Rod, look at the picture in Mike's Wikipedia link in the techniques section. Skins were originally made of seal skin, but now are simply a fabric with the nap set such that the skis will slide forward, but not backwards. The skins usually loop over the tip of the ski, and have a very sticky adhesive surface so they adhere to the bottom surface of the skis.
Skins work better than the fish scale pattern imprinted on the bottom of x-c skis, so people can go up steeper slopes. But they still have their limits. At that point, people would need to resort to snow shoes or crampons.
